This invention relates generally to magnetic systems for adjusting the beam positions in a multiple-beam cathode ray tube, and more particularly to an arrangement for the complex magnetization of magnetic rings used to effect static convergence of the plurality of electron beams in an in-line, tri-beam shadow-mask color kinescope.
In a kinescope of this type, the electron beams are aligned to originate beam paths having axes lying essentially in a common plane, with a central beam oriented in registry with the tube neck axis and with respective outer beam paths symmetrically disposed on opposite sides of the central beam.
While the electron guns are designed to direct the three beam paths to strike coincident regions of the phosphor screen after they pass through the openings in the shadow mask in the absence of applied beam deflection, in commercial manufacturing practice it is nearly impossible to prevent the introduction of misconvergence errors which require the presence in the kinescope of some means to correct these errors.
Adjustable magnetic fields produced by individually adjustable permanent magnets, or electromagnets, have been employed for use with both in-line and delta gun configurations to produce the complex magnetic field patterns necessary to effect the requisite static convergence adjustments of the electron beams.
In U.S. Pat. No. 3,725,831, there is disclosed one form of static convergence system for an in-line tri-beam kinescope which consists of three pairs of flat ring-shaped magnets which, for convenience are supported on the exterior of the kinescope neck for individual rotation about the neck axis. One pair of juxtaposed rings are magnetized to provide six poles symmetrically positioned about the ring periphery and alternating in polarity, i.e. with reference to a given north pole location, the remaining pole locations are: S-60.degree.; N-120.degree.; S-180.degree.; N-240.degree. and S-300.degree.. A second pair of rings is magnetized in a quadripolar arrangement symmetrically positioned about the ring periphery and alternating in polarity, i.e., with reference to a given north pole location, the remaining pole locations are: S-90.degree.; N-180.degree.; and S-270.degree.. A third pair of rings is magnetized in a symmetrical bipolar arrangement about the periphery of the ring.
Conjoint rotation of the rings of a pair alters the direction of the resultant beam shifts while differential rotation of the rings of a pair alters the beam shift magnitude. Rotation of the quadripolar and sextipolar rings has no effect on the central beam since this region in the case of these rings is substantially field-free. Rotation of the quadripolar rings produces shifts of the two outer beams in equal but opposite directions, while rotation of the sextipolar rings produces equal shifts of the outer beams in the same direction. Finally, rotation of the bipolar rings causes all three beams to shift in same direction in equal amounts. As stated above, the extent of these shifts can be controlled in each case by angular displacement of one ring of a pair with respect to the other ring of the same pair.
An improvement over this basic system, in which only a single magnetic ring is required, is disclosed in West German Offenlegungsschrift No. 26 11 633. In this disclosure a single ferromagnetic ring is put in place concentric with the central beam and either within, or without, the neck of the kinescope. An electromagnetic device having eight radially arranged symmetrically located poles is then arranged around the magnetic ring on the outside of the kinescope. The polarity and field strength of each of the eight poles can be individually controlled to algebraically produce a complex field which acts on the three electron beams in the same manner as is accomplished by the rotation of the several magnetic rings described in U.S. Pat. No. 3,725,831. When the appropriate current values and directions of current flow have been determined, these values can be used to actuate a magnetizing device to magnetize the magnetic ring installed in the kinescope to generate the complex magnetic field required to produce static convergence and purity of the three electron beams in that particular kinescope. The auxiliary device for performing the initial deflection of the beams can be connected to store the necessary information for operating the magnetizing device or can be used with a control device for automatically magnetizing the installed convergence ring and, after this has been performed, both the auxiliary device and magnetizing device are removed.
A further development for static convergence of electron beams is shown in West German Offenlegungsschrift No. 26 12 607, in which two axially spaced ring magnets of relatively low coercivity are placed closely surrounding the electron beams, one of the magnets surrounds the grids of the electron beam generating system and other is located near the lugs facing the picture screen which serve to center the generating system within the neck of the kinescope. The rings may be composed of wire having a diameter of only about 1.5 mm formed into rings of about 30 mm. in diameter and a suitable material consists of an alloy of Fe, Co, V, and Cr having a coercive field strength .sub.B H.sub.C of 24-32 kA/m. In this case the magnetization of the rings is accomplished by using a series of ferromagnetic rings provided with six, eight or twelve radially inwardly directed poles. Individually energized coils are wound on the sections of the ring between each pair of poles and the complex field magnetization is produced by separate control of the value and polarity of the current supplied to each of the coils. In one method the wire ring is first magnetized to saturation by means of a strong current pulse of the correct polarity to all of the coils and then demagnetized by pulses of opposite polarity and correctly adjusted values of current to each of the coils. Demagnetization can also be accomplished slowly with a 50 or 60 Hz alternating field until the optimum of increasing amplitude is reached.